I've also never really been clear about how smart the validators are. To
switch the scenario to stations I am familiar with, if one switches
between
tube and Thameslink at Farringdon, is one expected to touch-out on the
tube
platform and in on the NR one (or vice-versa) or is just one validation
on
either sufficient for the system to sort it all out. Similarly if
switching
between tube and Tramlink at Wimbledon, is validating on the Tramlink
platform sufficient for the system to register a touch-out from the tube
system?

Thameslink have notices up specifying in which area you can use
Oysters. Roughly speaking it is at stations between West Hampstead and
Elephant and Castle. Probably London Bridge as well, BICBW.

Yes, the question was not whether Oyster Pre-Pay could be used, but how it
is supposed to be used. There are validators on the Thameslink platforms
and at least one end of the tube platforms. If transferring between
Thameslink and tube using a Pre-Pay Oyster is one supposed to use the
validator on both Thameslink and tube platforms, or will just one or the
other suffice?